I can definitively tell you that it's not a good idea to use a custom CMS.
I've worked in a web agency in the past (late in 2006) where we've build a custom cms that we deploy for many client. From the beginning it was a really good idea because current developper really know the cms, we can do every thing we want in every way we want.
But, time passing, some developpers leaved, new one comes and we need to train them to our custom cms. We didn't open source it so new developper have to be very patient to learn it.
And it was really a lose time to train new developper. Also, each time we built a new feature, we need to write docs for new developper. And the cms started to be very huge, hard to maintain, provide doc, etc ..
At the end, all developpers from the first version had left the company. So basically, the company should rely on a CMS that nobody really knows. Ouch!
So we decide to switch to ezPublish.
In an other way, if you choose an open source CMS, you are almost sure that you're not the only one to know how it works. And you will have more developper than you ever dreamed to work on this project. It will also be more easy to find new developper that know the CMS. And you won't have to lose your time be training them.
A good example is Dailymotion. They started the company by create a custom framework. Few years later, they made the same statement I described above: maintain the framework was a pain in the ass. So they decide to migrate to Symfony 2 to have something more robust, etc ..
Over time, they started to feel the pain of maintaining their own framework: the maintenance of the framework itself started to take way too much time: adding new features, documenting them, training new developers, keep everything coherent, etc...
By choosing an Open-Source framework with a large community, they free themselves from everything other than their business.
You might also read this post.